Seismic waves are a very direct and quite clearly understood means of investigating the interior of the earth.
 
The following is distilled from 
THIS page on seismic waves in the earth.
 
p-waves are COMPRESSIONAL waves which can and do move through liquids. s-Waves (Shear waves) are incapable of moving through liquids.
 
p-wave velocity is simply modelled by this equation:
 
	
		
	
 
a=p-wave velocity
k=bulk modulus of the rock
u=shear modulus of the rock
p=density of the rock.
 
Simple, 4 terms explains things.
 
If the density or the moduli of the rock did not change with depth the speed would not change. Since we know how large the planet is we would be able to measure the difference between when a p-wave was generated on one side of the earth and when it was picked up on the other. We do this very thing and we see a couple of things:
 
1. p-waves follow and ARC through the mantle.
 
2. we know how long it takes for a p-wave to travel a known distance, so we know quite a bit about the density and moduli of the rock.
 
	
		
	
This arcing is caused by changes in density which REFRACT and bend the wave as it travels.
 
This is shown in many wave applications, and is similar to the effect known as SNELLS LAW (why a pencil put in a glass of water looks "bent")
 
	
		
	
This is a graph of the speed of the p and s-waves as they move down through the earth (remember, we see them when they come out on the other side or before they are occluded by the shadow zones
 
	
		
	
Note how the s-wave disappears at the inner core. This is because s-waves cannot travel through a liquid. Not just that they are "bent" but that they cannot travel through a liquid.
 
p-waves, however, can. And interestingly enough the fact that there is a layer of dense liquid is interestingly shown by a significant decrease in the p-wave velocity. It also bends here because of the abrupt change in density between the lower mantle and the outer core. This causes the p-waves to refract backwards, and results in the p-wave shadow zone.
 
THIS is a map of the s-wave SPEEDS at 2,880 km deep into the earth:
 
	
		
	
Just above the core-mantle boundary. We know this because we know WHERE the s-waves come from, and how far away they are picked up by seismometers (very sensitive instruments) nearly half-way around the earth!
 
We also know the limitations of our ability to measure this, note how we don't have s-wave data on the outer core...because it doesn't transmit s-waves. We need the p-wave data for that!
 
So we see very far into the earth.
 
Dad, if you are "honest" enough to debate any of this then you now have a very good place to start. Don't just tell me things are "different" down there. Tell me WHY you know this. Tell me what part of THE earlier equation is WRONG
 
Tell me how WAVES REFRACT in spiritual media such that they give data that matches the density, size, modulus and mineralogy of the mantle as we know it without it being a correlation.
 
You have everything you need here. If you need more, provide it.
 
Again, you have to address these points before you can make any others on this matter or I will have to conclude you are not honestly interested in debating the science.